The objectives of this three-year study are to (1) determine the relationship between chronic pain and immune function, and (2) examine the effects of depression and health behaviors as potential mediators between chronic pain and immunocompetence. More specifically, the aims of the study are: (1) to assess cross-sectionally at two times the relationship between chronic pain and immunocompetence; (2) to estimate two different components of pain (levels of pain and length of time with chronic pain) to selective immune function parameters at twoe times; (3) to quantify between group differences in the chronic pain group and a healthy non-pain control group at two times of selective immune function parameters; (4) to describe within group changes for all the subjects (chronic pain and controls) on immunocompetence as they occur throughout the course of the study; and (5) to estimate whether the relationship between chronic pain and immune function is mediated by depression and health behaviors. A prospective ex post facto design will be used to examine the effects of chronic pain on immunity. Seventy-five chronic back pain patients will be compared with 32 asymptomatic control subjects matched for age and sex to determine possible immune function differences. Each group will be followed for 30 days with 2 blood draws (Time 1 as baseline; Time 2, 30 days after baseline). Differences in immune function as well as levels of depression (measured by the Beck Depression Inventory), pain (measured by the short-form of the McGill Pain Instrument), and health behaviors (measured by the Personal Lifestyle Questionnaire) will be examined in all the subjects at each data collection period. Incidence of infectious illness will be monitored throughout the course of the study using the Health Review checklist. Data will be analyzed using repeated measures ANOVA and ANCOVA, multiple and logistic regression, and bivariate correlational techniques. Results of this study will strengthen the foundation of research that is exploring possible linkages between the chronic pain experience and immune function.